Known ion mobility spectrometers typically comprise a drift tube wherein ions are caused to drift under the influence of a constant applied electric field. Various constructions of drift tube have been proposed. The drift tube may, for example, comprise a series of ring electrodes axially spaced apart along the length of the spectrometer, wherein a constant potential difference is maintained between adjacent ring electrodes such that a constant electric field is produced in the axial direction. A pulse of ions is introduced into the drift tube, which contains a buffer gas, and as the ions travel through the tube under the influence of the constant electric field they attain a constant drift velocity and separate in the axial direction according to their ion mobility. The buffer gas is often arranged flowing in the opposite direction to the direction of ion travel.
An ion mobility spectrometer may be operated on its own as a means for ion separation or it may be used in combination with other ion separation devices in so-called hybrid IMS instruments. Examples of hybrid IMS instruments include those based on liquid chromatography IMS (LC-IMS), gas chromatography IMS (GC-IMS) and IMS mass spectrometry (IMS-MS). The latter type of instrument is a powerful analytical tool which employs mass spectrometry for further separating and/or identifying peaks in an ion mobility spectrum. More than two separation techniques may be combined, e.g., GC-IMS-MS.
Ion mobility spectrometers may be operable at atmospheric pressure (see e.g. U.S. Pat. No. 5,162,649) and can offer a resolution of up to 150 (see e.g. Wu et al., Anal. Chem. 1998, 70, 4929-4938). However, operation at lower pressures is more suitable for hybrid IMS-MS instruments (see e.g. U.S. Pat. No. 5,905,258 and WO01/64320) to increase speed of separation and reduce ion losses. Operation of the ion mobility spectrometer at lower pressures frequently leads to greater diffusion losses and lower resolution. In order to counter the problem of diffusion losses, an RF pseudo-potential well may be arranged in the drift tube to confine ions radially so that it acts as an ion guide and may be used to transportions efficiently (see e.g. U.S. Pat. No. 6,630,662).
In a modification to an ion mobility spectrometer, U.S. Pat. No. 6,914,241 describes how ions may be separated according to their ion mobility by progressively applying transient DC voltages along the length of an ion mobility spectrometer or RF ion guide comprising a plurality of axially spaced apart electrodes. The ion mobility spectrometer may comprise an RF ion guide such as a multipole rod set or a stacked ring set. The ion guide is segmented in the axial direction so that independent transient DC potentials may be applied to each segment. The transient DC potentials are superimposed on top of an RF voltage which acts to confine the ions radially and/or any constant DC offset voltage. The transient DC potentials thereby generate a so-called travelling wave which moves along the length of the ion guide in the axial direction and which acts to move ions along the length of the ion mobility spectrometer.
In the above types of ion mobility spectrometers, ions are propelled along the ion guide and ions may be separated according to their ion mobility. However, in order to achieve a high resolution or resolving power of ion mobility separation at relatively low pressures, a relatively long drift tube must be employed in order to keep within the so-called low field limit as described in more detail below.
In order to separate ions along the axial direction according to their ion mobility in an RF ion guide, an axial DC electric field may be generated which is orthogonal to the radial RF field for radial confinement. If a constant axial electric field E is applied in order to move ions along and through an ion guide containing a gas, then the ion will acquire a characteristic velocity, v according to:v=E*K  (eqn. 1)
wherein K is the ion mobility.
In order to maintain ion mobility separation in the so called low field regime wherein ions do not receive significant kinetic energy from the driving field, the ratio of E (in V/m) to the pressure of the background gas P (in mbar) should be maintained at a value less than about 200V/(m*mbar). At the same time, resolving power, R, of separation according to ion mobility (FWHH) is limited by diffusion and can be approximately estimated as:
                    R        =                              1            2                    ⁢                                                    e                ⁢                                                                  ⁢                z                ⁢                                                                  ⁢                E                ⁢                                                                  ⁢                L                                            k                ⁢                                                                  ⁢                T                                                                        (                  eqn          .                                          ⁢          2                )            
wherein z is the charge state of ions, L is the length of separation (m), T is temperature (degrees Kelvin) of background gas, e is the elementary charge (1.602*10−19 Coulomb) and k is Boltzmann's constant (1.38*10−23 J/K). More accurate calculations can be found, e.g., in G. E. Spangler, “Expanded Theory for the resolving power of a linear ion mobility spectrometer”, Int. J. Mass Spectrom. 220 (2002) 399-418. As increase of E is limited by low-field conditions and decrease of T is associated with cumbersome cryogenic techniques, it can be seen that the only way towards achieving higher R is to increase the separation length L. However, increasing the separation length can be problematic since space is typically limited.
One solution to the problem of increasing the separation length proposed in the prior art of WO2008/104771, GB2447330 and GB2457556 is to coil the ion mobility drift tube. However, construction of the drift tube becomes complex in that case and precludes rapid transfer of ions through the spectrometer in the case when no mobility separation is required.
It can therefore be seen that there is a need to improve ion mobility spectrometers, particularly a need to provide an ion mobility spectrometer having an increased separation length and more particularly a need to provide an ion mobility spectrometer having an increased separation length but without complex construction. In view of the above background, the present invention has been made.